1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, and particularly to a skew feed correcting portion for correcting the skew feed of a sheet.
2. Description of the Related Art
In an image forming apparatus such as a printer, a copying machine or a printing machine, a toner image or an ink image is formed on a conveyed sheet by an electrophotographic printing method, an offset printing method, an ink jet printing method or the like.
A color image forming apparatus using, for example, the electrophotographic printing method is classified, from its construction, chiefly into a tandem type in which a plurality of image forming portions are juxtaposed, and a rotary type in which a plurality of image forming portions are disposed in a cylindrical shape. Also, from its transferring method, it is classified into a direct transfer type in which a toner image is directly transferred from a photosensitive drum which is an image bearing member to a sheet, and an intermediate transfer type in which a toner image is once transferred to an intermediate transfer member, and thereafter is transferred to a sheet.
Here, in an intermediate transfer tandem type wherein image forming portions of four colors are juxtaposed on an intermediate transfer belt, it is not necessary to hold a sheet on a transfer drum or a transfer belt as in the direct transfer type. Therefore, a color image forming apparatus adopting the intermediate transfer tandem type can cope with a variety of materials such as super-thick paper and coat paper and moreover, is suitable for the realization of high productivity because of its features such as the parallel processing in the plurality of image forming portions and the collective transfer of a full-color image.
Now, in recent years, in the field of such an electrophotographic printing apparatus, there have been provided apparatuses which make the most of the merit of making no print, and aims, for example, at a print-on-demand (POD) market for effecting a small number of prints. However, in order to be accepted by such a quick printing market, it is necessary to achieve a high quality of image, and as an important factor for achieving the high quality of image, there is an image position to a sheet. This image position accuracy includes the deviation of the front side and back side when two-side images have been formed.
When the image position accuracy to the sheet is to be improved, it is necessary to improve the accuracy of the registration in the conveyance direction of the sheet, the registration in a direction of the sheet, an image magnification, skew feed correction, etc.
Here, the registration in the conveyance direction, the registration in the direction orthogonal to the conveyance direction and the magnification can be corrected by electrical control, but the skew feed in difficult to correct by the electrical control.
For example, it is possible to detect the skew feed of a sheet, and form an image inclined in accordance therewith to thereby correct the image position to the sheet. However, particularly in the case of a color image in which three or four colors are superposed one upon another, when an image is inclined on each sheet, the hue is changed for each sheet by the deviation of the dot formation of each color. Also, much time is required for the calculation for inclining the image, thus resulting a remarkable reduction in productivity. Consequently, the correction of the skew feed is determined by the performance of the conveyance accuracy of the sheet.
So, heretofore, a skew feed correcting portion for correcting the skew feed of a sheet has been provided in a sheet conveying apparatus for conveying the sheet to a transferring portions, so as to correct the skew feed of the sheet. Here, a method of correcting the skew feed of the sheet is divided broadly into the following four methods.
1. A pair of registration rollers are disposed upstream of the transferring portions, and the leading edge of the conveyed sheet is rammed against the registration rollers being at a halt to thereby push in the sheet from the trailing edge side thereof, and form a loop, thereby effecting skew feed correction. Thereafter, the registration rollers are re-started in synchronism with the image, to thereby effect the skew feed correction and the image adjustment in the conveyance direction of the sheet.
2. By the use of a retractable shutter instead of the registration rollers, the leading edge of the conveyed sheet is rammed against it is thereby form a loop, thus effecting skew feed correction. Thereafter, on the basis of the detection of the leading edge of the sheet by a sensor, the position adjustment with the sheet is effected, and skew feed correction and the image adjustment in the conveyance direction of the sheet are effected.
3. The sheet is rammed against a conveyance reference wall provided in the conveyance direction of the sheet by a obliquely feed roller and is conveyed at the same time to thereby correct the skew feed of the sheet, and the image position adjustment in the conveyance direction is effected by detecting the leading edge of the sheet by a sensor and controlling the conveyance speed of the sheet (disclosed, for example, in Japanese Patent Application Laid-open No. H11-189355).
4. Provision is made of means for detecting the skew feed of the sheet and two drive rollers capable of driving independently in a direction perpendicular to the conveyance direction of the sheet, and the speeds of the respective drive rollers are changed in accordance with the skew feed amount of the sheet to thereby correct the skew feed (disclosed, for example, in Japanese Patent Application Laid-open No. H05-201587).
Here, the third method, when viewed regarding the deviation of the front side and back side of the images during two-side printing, has the merit that although the sheet is switched back, whereby the leading edge and the trailing edge change places with each other between a first side and a second side, the sides do not change places with each other and therefore, skew feed can be corrected on the same reference for both of the first side and the second side. In the other methods, skew feed correction is effected at the leading edge and therefore, the reference for effecting correction differs between the first side and the second side and thus, when the parallelism between the leading and trailing edges of the sheet is not sufficient, the deviation between the front side and the back side will occur even if the correction capability is high.
FIG. 16 of the accompanying drawings is a plan view showing the construction of a skew feed correcting portion which corrects skew feed by such a third method. This skew feed correcting portion is provided in a sheet conveying apparatus for conveying a sheet on the so-called center reference.
In FIG. 16, a obliquely feed roller guide portion 100 is provided with a side regulating plate 105 and obliquely feed rollers 253, 254 and 255. These obliquely feed rollers 253, 254 and 255 are inclined and provided on a lower guide 102 side so as to convey the sheet toward the side regulating plate 105. When the sheet S is conveyed by the obliquely feed rollers 253, 254 and 255, the sheet S is rammed against a reference wall 105a provided on the side regulating plate 105 shown in FIG. 18 of the accompanying drawings which will be described later, and has its skew feed corrected thereby.
The reference numeral 115 designates a fixed lower guide constituting a sheet conveying path together with the obliquely feed roller guide portion 100, which is movable relative to the fixed lower guide 115 in a width direction orthogonal to the conveyance direction of the sheet.
Here, the obliquely feed roller guide portion 100 is movable in accordance with the size of the sheet S, and is moved in advance to a position corresponding to the size before the sheet S is conveyed thereto, thereby contriving the higher efficiency of the correction time. When the skew feed of a sheet Smax of a maximum size is to be corrected, as shown in FIG. 16, the gap between the obliquely feed roller guide portion 100 and the fixed lower guide 115 becomes a maximum Gmax. Also, when the skew feed of a sheet 5 min of a minimum size is to be corrected, as shown in FIG. 17 of the accompanying drawings, the gap between the obliquely feed roller guide portion 100 and the fixed lower guide 115 becomes a minimum Gmin. The wider is the range of the passable sheet size, the greater becomes the gap between the obliquely feed roller guide portion 100 and the fixed lower guide 115 when the sheet of the maximum size is passed.
Now, the sheet conveying apparatus provided with such a conventional skew feed correcting portion suffers from the following problems. The greater is the difference between the maximum size and minimum size of conveyable sheets, the wider becomes the gap between the obliquely feed roller guide portion 100 and the fixed lower guide 115 when the skew feed of the sheet of the maximum size is corrected. When the gap between the guides thus becomes wide, downward flexure indicated by arrow P at the central portion as shown in FIG. 18 is caused to occur to the sheet S by its gravity.
When the sheet S is conveyed with the downward flexure thus caused to occur to the sheet, as shown a side view of in the apparatus of FIG. 19 of the accompanying drawings, the central portion of the sheet S is sometimes caught by the entrance lower guide 300b of a pair of registration rollers 131 and 132 provided downstream of a lower guide 102 and the fixed lower guide 115. When the sheet S is thus caught, jam is caused or bending occurs to the central portion of the sheet. This tendency is more remarkable in the case of thin sheets of weak stiffness.